The result of all radiographic methods such as, for example, computed tomography, mammography, angiography, X-ray inspection technology or comparable methods is the representation of the attenuation of an X-ray beam along its path from the X-ray source to the X-ray detector. This attenuation is caused by the transradiated media or materials along the beam path. The attenuation is usually defined as the logarithm of the ratio of the intensity of the attenuated radiation to the primary radiation, and denoted, with reference to the path normal, as attenuation coefficient of the material.
A multiplicity of radiographic examination units use not the attenuation coefficient, but a value normalized to the attenuation coefficient of water, the CT number, in representing the attenuation distribution of an X-ray beam in an object to be examined. This number is calculated from an attenuation coefficient μ currently determined by a measurement, and the reference attenuation coefficient μH2O according to the following equation:
                    C        =                  1000          ×                                                    μ                -                                  μ                                                            H                      2                                        ⁢                    O                                                                              μ                                                      H                    2                                    ⁢                  O                                                      ⁢                                                  [            HU            ]                                              (        1        )            where the CT number C is expressed using the Hounsfield unit [HU]. A value of CH2O=0 HU results for water, and a value of CL=−1000 HU for air.
Since both representations can be transformed into one another, or are equivalent, the generally selected term of attenuation value denotes both the attenuation coefficient μ and the CT value in what follows. Furthermore, the terms of material and tissue are used interchangeably in the substantive context of this description of the embodiments of invention. It is assumed that a material within the context of a medically indicated examination can be an anatomical tissue, and, conversely, that in testing of materials and in safety testing a tissue is to be understood as any desired material of an object to be examined.
Increased attenuation values can be ascribed either to materials of higher atomic number such as, for example, calcium in the skeleton or iodine in a contrast medium, or to an increased density of soft parts such as, for example, in the case of a lung nodule. The local attenuation coefficient μ at the location {right arrow over (r)} is a function of the X-ray energy E irradiated into the tissue or material, and of the local density ρ of tissue or material in accordance with the following equation:
                    μ        =                              μ            ⁡                          (                              E                ,                                  r                  →                                            )                                =                                    (                              μ                ρ                            )                        ⁢                          (              E              )                        ×                          ρ              ⁡                              (                                  r                  →                                )                                                                        (        2        )            with the mass attenuation coefficient
      (          μ      ρ        )    ⁢      (    E    )  dependent on energy and material.
The energy-dependent X-ray absorption of a material, as is defined by its effective atomic number, is thus superimposed on the X-ray absorption that is influenced by the material density. Materials or tissues of different chemical or physical composition may thus have identical attenuation values in the X-ray image. Conversely, on the other hand, it is impossible to deduce the material composition of an object to be examined from the attenuation value in an X-ray picture.
Correct interpretation of the distribution—thus actually rather unclear—of the X-ray attenuation values in an X-ray image produced using a radiographic examination method can generally be carried out only on the basis of morphological criteria in the medical sector, and generally requires a radiologist with decades of experience in his field. Nevertheless, in some circumstances, structures which occur with increased attenuation values in the imaging process for an X-ray examination cannot be clearly classified. For example, it is difficult to distinguish between calcification close to the hilus on a thorax overview picture and a vessel which is located orthogonally with respect to the imaging plane. It is also virtually impossible to distinguish, for example, between diffuse calcification and fresh bleeding.
Even in the case of materials testing and safety testing, the tester generally supplements the information in the display of an attenuation value distribution by his personal specialist knowledge and professional experience. Nevertheless, it is impossible, for example, for him to distinguish reliably between plastic-bonded explosive mixtures and a non-explosive plastic directly from an X-ray image.
Methods for displaying material-characteristic values are required for this purpose. In “Materialselektive Bildgebung und Dichtemessung mit der Zwei-Spektren-Methode, I. Grundlagen und Methodik, W. Kalender, W. Bautz, D. Felsenberg, C. Süβ und E. Klotz, Digit. Bildiagn. 7, 1987, 66–77, Georg Thieme Verlag” [“Material-selective imaging and density measurement with the aid of the two-spectra method, I. Fundamentals and Methodology, W. Kalender, W. Bautz, D. Felsenberg, C. Süβ and E. Klotz, Digit. Bilddiagn. 7, 1987, pages 66–77, Georg Thieme Verlag”], W. Kalender et al. describe a method for base material decomposition in the case of X-ray pictures. The method is based on the effect that materials and tissues of higher atomic number absorb low-energy X-radiation substantially more intensely than do materials or tissues of lower atomic number. By contrast, in the case of higher X-ray beam energies the attenuation values become assimilated and are largely a function of the material density.
Unless otherwise indicated, in the context of this description the term of atomic number is not used in the strict sense relating to the elements, but instead denotes an effective atomic number of a tissue, or material, that is calculated from the chemical atomic numbers and atomic weights of the elements which are involved in the formation of the tissue or material.
In the method proposed by W. Kalender et al., the X-ray attenuation values of an object to be examined are measured with the aid of X-ray beams of lower and higher energy, and the values obtained are compared with the corresponding reference values of two base materials such as, for example, calcium (for skeletal material) and water (for soft part tissues). It is assumed that each measured value can be represented as a linear superposition of the measured values of the two base materials. For example, a skeletal component and a soft tissue component can be calculated for each element of the pictorial display of the object to be examined from the comparison with the values of the base materials, the result being a transformation of the original pictures into displays of the two base materials of skeletal material and soft part tissue.
The base material decomposition or the two-spectrum method is therefore suitable for distinguishing between anatomical structures in human and animal tissues with a strongly differing atomic number. In materials testing and safety testing, it would therefore be possible, for example, to distinguish according to predefined types of materials, so-called material classes. The aim of the base material decomposition is not a functional display that permits detection of the physical and chemical characteristics of the materials examined, or variations in these characteristics within a type of material.